1. Field of the Invention
The present invention relates to a control for correcting a toner amount of a toner image formed in an image forming apparatus that adopts an electrophotographic system.
2. Description of the Related Art
In a full-color image forming apparatus that adopts an electrophotographic system, electrostatic latent images formed on a photosensitive member are developed by using developer including toner of respective color components to form toner images of the respective color components. These toner images of the respective color components are overlapped and transferred in a transfer unit so that a full-color toner image in accordance with an original is formed. This toner image is fused on a sheet such as paper by applying heat and pressure in a fusing unit and thereafter output from the image forming apparatus as a printed product.
A density of the toner image formed by this image forming apparatus is decided by the amount of toner per unit area in the formed toner image.
However, when toner images of a plurality of color components are overlapped to form a full-color toner image, if the amount of toner in the overlapped and formed toner image (the amount of toner per unit area) exceeds an upper limit amount, the mutual toners charged to a homopolarity repel each other, and scattering of the toner occurs. Furthermore, when the toner image is fused on the sheet, the amount of toner in this overlapped toner image is too large to be fused, the toner scatters by the heat and the pressure applied from the fusing unit.
This scattering of the toner is particularly likely to occur in a part for a line drawing or a character. When the scattering of the toner occurs in the part corresponding to the line drawing, a line drawing thicker than a line width of the original image is formed. If the scattering of the toner occurs in a part corresponding to the character, the character cannot be read, or it becomes difficult to read the character.
According to Japanese Patent Laid-Open No. 2006-98473, the toner amount of the toner image of the line drawing or the character is predicted from a density detected from a toner image for a density detection, and an upper limit value of a signal level at a part for the line drawing or the character is decided from this prediction result. At this time, like the toner image corresponding to the area for the line drawing or the character, the toner amount of the toner image for the density detection at a rear end part in a direction in which the toner image is conveyed is increased since a development field concentrates. For that reason, as the value of the density of the toner image for the density detection at the rear end part in the direction in which the toner image is conveyed is higher than the value of the density at the central part in this conveyance direction, the toner amount in the area for the line drawing or the character is also increased. With this configuration, according to Japanese Patent Laid-Open No. 2006-98473, by correcting the signal level in the area for the line drawing or the character to have a value equal to or smaller than the upper limit value decided from the prediction result, it is possible to suppress the toner amount of the toner image of the line drawing or the character formed in accordance with this corrected signal level.
However, even when the signal level in the area for the line drawing or the character is corrected to have a value equal to or smaller than the upper limit value by using the method described in Japanese Patent Laid-Open No. 2006-98473, a problem exists that the scattering of the toner occurs in the part for the line drawing or the character since an environment is changed or a large number of images are formed.
This is because the toner image bearing the toner at a narrow width smaller than or equal to a predetermined width tends to have a larger toner amount than the rear end part of the toner image bearing the toner in an area wider than the predetermined width due to the degradation of the developer since the temperature or the humidity is changed or a large number of images are formed. For that reason, in a case where the areas of the line drawing and the character are formed, even when the signal levels of these areas are corrected to be smaller than or equal to the upper limit value predicted from the value of the density, the actually formed toner image has the toner amount larger than the amount of the upper limit at which the scattering of the toner can be suppressed.